Hybrid drive vehicles are powered by two or more energy sources. Typical hybrid drive vehicles include a heat engine and a battery connected to a motor/generator as energy sources. Other hybrid drive vehicles may, for example, employ a fuel cell in hybrid combination with a battery.
An engine or fuel cell in a vehicle without hybrid drive acts as the sole source of energy to propel the vehicle. Accordingly, the behavior of the engine or fuel cell is directly related to the power applied to the vehicle's drive wheels to accelerate or maintain a given speed. A driver associates changes in engine sounds or fuel cell compressor sounds with changes in power delivered to the drive wheels for acceleration, climbing a hill, etc.
The flow of energy between vehicle components in a hybrid drive vehicle may be confusing to a driver because, in a hybrid drive vehicle, the behavior of the energy sources may not directly correspond with vehicle acceleration or speed. For example, when an engine or fuel cell is transmitting energy to charge a battery, a driver may be confused by changes in engine or fuel cell compressor sounds without a corresponding change in acceleration or power necessary to maintain speed.